<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .btn1 {
            width: 100px;
            height: 50px;
            background-color: greenyellow;
            border: 1px solid black;
            padding: 10px;
            margin: 20px auto;
            display: block;
            border-radius: 5px;
        }
    </style>
    <script src="js/demo.js" type="text/javascript"></script>
</head>
<body>
    <button class="btn1" onclick="showAlert()">点开我</button>
</body>
</html>